A relatively new combustion strategy known as homogeneous charge compression ignition (HCCI) shows great promise in reducing undesirable emissions from internal combustion engines that utilize a compression ignition strategy. HCCI refers generally to the idea of mixing fuel with air in the engine cylinder before autoignition conditions arise. The mixture is compressed to autoignition, with a general desirability that the combustion event take place in the vicinity of top dead center. Although HCCI has proven the ability to drastically reduce undesirable emissions, especially NOx, the combustion strategy has brought new problems that must be overcome in order to render such an engine commercially viable. For instance, for an HCCI engine to be a viable alternative to a counterpart conventional diesel engine, it must have the ability to operate effectively over a relatively wide load range. One problem encountered with HCCI engines is the extreme sensitivity and difficulty in controlling ignition timing. In addition, HCCI engines can have difficulty in operating in higher load ranges where more fuel is supplied to the individual cylinders. This perceived limitation may be due to extreme pressure spikes that occur when the charge burns. The pressures can rise so fast and get so high as to exceed the vibrational and structural containment capability of an engine housing. Thus, operating an HCCI engine, especially at higher speeds and loads, can be extremely problematic, but must be overcome to enable such an engine to be a viable alternative to a counterpart conventional diesel engine.
Apart from problems associated with high load operation, multi-cylinder HCCI engines have even more problems that need to be overcome. For instance, engine geometry, including the intake geometry, fuel injector performance variations, and other known and unknown influences affect the specific burn behavior of charges in different ones of a plurality of engine cylinders. For instance, intake manifold geometry may result in one cylinder receiving less or more air than other cylinders, and a fuel injector performance variation in another cylinder may cause less or more fuel to be injected based upon the same control signal. These differences result in different air/fuel ratios in different cylinders. HCCI combustion is very sensitive to air/fuel ratio and other factors including charge mass, compression ratio, as well as the initial temperature and pressure of the charge when compression begins. Thus, these variations can contribute to substantial differences in both heat release and combustion phase timing among a plurality of different engine cylinders. While some variation may be acceptable at lower load conditions, at higher speeds and loads, combustion phasing and heat release variations among the plurality of cylinders can give rise to unacceptable noise levels. In addition, assuming equal air/fuel ratios, the cylinder with the most advanced combustion timing will develop a maximum pressure and pressure rise rate at a lower load, and therefore will limit peak load of the entire engine. Those skilled in the art will appreciate that, when the charge burns too early in the combustion stroke, excessive cylinder pressures and pressure rise rates can occur that limit the load carrying capability of that cylinder, and hence the entire engine.
In addition to the above problems associated with ignition timing, controlling peak pressures and pressure rise rates, and balancing combustion among a plurality of engine cylinders, there are even more problems when there is recognition that the engine must not only have the capability of operating in a steady state condition, but also must have the capability of transitioning among different combinations of speed and load. This problem is further compounded by recognition that different control strategies are so cross-coupled in their influence on engine behavior that no known control strategy is available for an HCCI engine that allows the engine to retain stable operation across a wide range of engine speeds and loads. In addition, some potential control techniques, such as varying exhaust gas recirculation rates and/or varying geometric compression ratio in the engine, may be too sluggish to react on an engine cycle to cycle basis. As used in this text, the term “geometric compression ratio” means the ratio of the cylinder volume when the engine piston is at bottom dead center to the volume of the cylinder when the piston is at top dead center. If the engine is too sluggish in transitioning from one combination of speed and load to another combination of speed and load, that behavior could also render it not viable as an alternative to a convention diesel engine.
Other potential control actuators may include fuel injection timing and quantity control, as well as variable intake and/or exhaust valve timing control. While these types of control actuators may have the ability to adjust from one engine cycle to the next engine cycle, a variable valve actuator alone may not provide a sufficient bandwidth of control to enable the engine to operate across a wide range of speeds and loads. For instance, a variable valve actuator, such as a variable intake valve actuator may not have a bandwidth that permits the engine to operate beyond a narrow range of speeds and loads without some other additional strategy. In addition, adjustments to variable intake valve actuators not only changes the compression ratio, but also the charge mass and the initial cylinder charge pressure and temperature. These factors interact to determine when auto-ignition conditions arise, if at all, in the engine cylinder.
In addition, further problems can occur by recognizing that control actuators may not actually be doing what the engine controller expects. For instance, if exhaust gas recirculation is controlled, even slight differences between the expected exhaust gas recirculation rate and the expected or desired rate can result in substantial variations in engine behavior. Thus, some of the control actuators themselves may need their own separate feedback control in order to be realistically utilized in the highly combustion-sensitive environment of a homogeneous charged compression ignition engine. Thus, the limited bandwidths of available control actuators, their interactions in altering combustion behavior, and uncertainties in the control actuators doing what the engine controller expects renders the problems associated with dynamic control of an HCCI engine nearly insurmountable.
One strategy for dealing with the problems of HCCI is described in co-owned U.S. Pat. No. 6,725,838. This reference describes a mixed mode strategy where HCCI is employed over a lower load range portion of the engine, and conventional diesel engine strategies are employed at high speeds and loads. By conventional, this disclosure means that fuel is injected directly into an engine cylinder after autoignition conditions have arisen, with the injection taking place in the vicinity of top dead center. While a mixed mode strategy can produce superior emissions compared to that of a conventional diesel engine, it requires increased complexity, especially in manufacturing and controlling the fuel system, but mixed mode still results in more undesirable emissions than that possible with an entirely HCCI operation strategy.
The present disclosure is directed toward one or more of the problems set forth above.